1. Field of the Invention
The present invention relates to a position displacement detecting apparatus suitable for a recording/reproducing apparatus and the like, using, for example, the principle of a scan type tunnel microscope.
2. Related Background Art
There is a tendency that the capacity of data recorded by a recent recording apparatus is increasing more and more. As the capacity increases, it is essential that the dimension of a record unit becomes correspondingly small and the record density becomes correspondingly high. For example, the dimension of a record unit of a digital audio disk of optical recording type is now in the order of 1 .mu.m.sup.2.
A scan type tunnel microscope (hereinafter abbreviated as STM) has been recently developed with which the electron structure of a material surface or near the surface can be directly observed [refer to G. Binnig et al., Helvectica Physica Acta, 55, 726(1982)]. With STM, it is possible to measure a real space image irrespective of monocrystalline or amorphous, with high resolution. Furthermore, it is possible to observe at low power without damaging a medium by a current flowing through it. Still further, STM can operate not only under super high vacuum but also under atmosphere or solution, allowing to apply it to various materials. Wide applications have thus been expected.
STM operates basing upon the fact that if a metal probe and conductive material become as close as about 1 nm and a voltage is applied therebetween, a current will flow therebetween. This current is very sensitive to the distance change between the probe and the conductive material. Therefore, the surface information of a real space can be obtained by scanning the probe while maintaining constant a current or the average distance therebetween. In this case, the resolution in an inplane direction is equal to or larger than one angstrom.
It is possible to record information on a recording medium using the principle of STM. In this case, a record unit of 0,001 .mu.m.sup.2 or smaller can be recorded by using as the recording medium a material having a memory function responding to voltage/current switching characteristics, such as a thin film layer made of .pi. electron organic compound or chalcogenide.
A voltage in excess of a certain threshold value is applied to a probe. Then, the characteristic of a fine area of a recording medium immediately under the probe changes, thereby allowing to record data. By using the fact that a tunnel current flowing between the probe and the recording medium changes between the recorded area and the unrecorded area, reproducing data becomes possible.
Similar recording/reproducing is possible by using as a recording medium a metal thin film such as Au and Pt whose surface is locally melted or evaporated to make protrusions and recesses upon application of a voltage in excess of a certain threshold voltage.